This invention relates to an electromagnetic detection system for use in a variable area fluid flowmeter, More specifically, the invention relates to a device for measuring the rate of fluid flow by detecting the position of a magnetic field created by a magnet displaceable by a fluid flow inside a vertical pipe.
In the prior art, a variable area fluid flowmeter consists of two parts, one part inside the pipe for producing physical displacement of a magnet, and another part outside the pipe for detecting the field produced by the magnet and measuring the magnet's distance from a reference position.
Prior art arrangements usually consist of a vertical pipe fitted with a horizontal orifice plate having a central round orifice. A conic plug is placed inside the pipe, the plug has a vertical pin for vertical guidance of the plug. The pin slides through two rings which are affixed to the pipe and restricts motion of the plug within the pipe to a vertical direction. The plug is free to slide up and down in the pipe. When there is no fluid flow, the conic plug rests in the central orifice. When there is fluid flow, the conic plug slides upwards until the open area between the plug and the orifice is large enough so that the gravitational force on the plug is equal to the force induced by fluid flow. The conic plug contains a bar magnet that produces a magnetic field which extends outside the pipe. The field which extends beyond the pipe is detected by various techniques.
One detection technique involves placing a second bar magnet outside the pipe. The second magnet is placed near the conic plug with its poles opposite to the poles of the magnet in the conic plug. The second bar magnet is pivotably mounted and movement of the conic plug causes the second magnet to rotate. Rotation of the second bar magnet is detected with means such as a variable resistor. The displacement of the bar magnet inside the conic plug is calculated from the rotation of the second bar magnet. This technique is cumbersome and requires added mechanical elements which are less reliable than electrical components.
Another prior art sensing technique involves placing multiple solid state electrically resistive elements in the external magnetic field. The resistance of these elements varies as a function of the magnetic field. By measuring the resistance, the displacement of the conic plug can be calculated. The resistive elements require a significant amount of power, which is added to the power required to perform the calculations to determine the position of the conic plug.
There is accordingly a need in the art for a sensing arrangement which requires no moving parts and which consumes low power.